This invention relates generally to waveguide couplers, and more particularly to a coupler between a rectangular waveguide and circular waveguides.
Gyrotron traveling-wave amplifiers, such as disclosed in U.S. Pat. No. 4,224,576 issued Sept. 23, 1980 to V. L. Granatstein et al., comprise an electron gun whose electron beam passes through a circular waveguide which is proportioned to support the circular TE.sub.01 mode to the exclusion of all of the higher-numbered circular modes. Microwave energy is introduced into the waveguide in the same direction as the electron beam by means of a coupler positioned in the gap between the electron gun and the circular waveguide. The input to the coupler is a rectangular waveguide supporting the dominant TE.sub.10 mode. A solenoidal winding surrounding the electron gun and the circular waveguide produces a longitudinal magnetic field whose strength increases in the gap region. Desirably, the width of the gap is kept small in order to provide an optimum magnetic field profile.
In the past, the coupler was unable to provide efficient coupling of microwave energy over a wide bandwidth because of the constraints placed upon it by the dimensions of the gap. In a practical embodiment of the gyrotron traveling-wave amplifier at 35 GHz the gap is only 0.7 inches wide. Typical slot or multi-hole directional couplers disclosed, for example, in the text Microwave Engineering by A. F. Harvey, Acedemic Press, 1963, have a good bandwidth but in order to provide efficient (zero-decibel) coupling they must be many wavelengths long. U.S. Pat. No. 2,960,670 issued to Marcatelli on Nov. 15, 1960, discloses a circular mode directional coupler of wideband and compact design. The drawback is, that in order to use it, a very long rectangular-to-circular taper must be used (many wavelengths long) and the rectangular-to-circular taper would have to be in the position of the electron gun in order to couple microwaves in the proper direction.